Injection molding machine with high-frequency dielectric heater

ABSTRACT

A system for dielectrically heating primarily glass fiber filled polyester materials in an indexing-type injection molding machine. The machine is indexable between a shot-receiving and shot-ejecting position and several forms of dielectric apparatus are shown for heating the shot to achieve a partial cure in one or more of these positions, or in an advance-receiving arrangement. In at least one species of the invention correlation between position of the shot in the molding machine and operation of the dielectric heater is automatically effected to achieve a high efficiency of operation. A system for controlling the operational steps of the molding machine is disclosed, a portion of the sequencing being dependent upon a predetermined level of heating of the shot material.

United States Patent Schwartz 5] Feb. 8, 1972 [72] Inventor: William H.Schwartz, University Heights,

Ohio

[73] Assignee: Lester Engineering Company, Cleveland,

Ohio

[22] Filed: Dec. 23, 1969 [21] Appl.No.: 868,726

US. Cl ..425/l74, 425/ l 44, 425/243 MacMillin... ...l8/30 FJ MaCMillin1 8/30 F] Primary Examiner-J. Spencer Overholser AssistantExaminer-Norman E. Lehrer Attorney0berlin, Maky, Donnelly & Renner [57]ABSTRACT A system for dielectrically heating primarily glass fiberfilled polyester materials in an indexing-type injection moldingmachine. The machine is indexable between a shot-receiving andshot-ejecting position and several forms of dielectric apparatus areshown for heating the shot to achieve a partial cure in one or more ofthese positions, or in an advancereceiving arrangement. in at least onespecies of the invention correlation between position of the shot in themolding machine and operation of the dielectric heater is automaticallyeffected to achieve a high efiiciency of operation. A system forcontrolling the operational steps of the molding machine is disclosed, aportion of the sequencing being depen dent upon a predetermined level ofheating of the shot materi 4 Claims, 8 Drawing Figures PAIENTEBFEB 8 m23.640.662

SHEET 1 []F 2 INVENTOR. WILL/14M H. SCHWARTZ I20 [7 ATTORNEYS INJECTIONMOLDING MACHINE WITH HIGH- FREQUENCY DIELECTRIC HEATER This inventionrelates generally to injection molding machines and more particularly toapparatus for applying dielectric heat to material in an indexingshot-type injection molding machine.

One of the problems encountered with the molding of polyester-typematerials having a glass fiber filler is the ability to producehigh-impact strength products in a reasonable amount of time. Althoughinjection molding with a screw-type machine has been employed in thepast, this technique presents problems with the glass fiber filledmaterials in that excessive working of the material produces adisorientation of the glass fiber strands from that distributionachieved in a premixed batch. This has resulted in widespread use of thecompression molding technique wherein batches of slugs are prepared forthe molding process but such technique is also limited in that the benchlife of such batches places a limit on the quantity of material that canbe produced and the rate of production of same.

The shot-type injection molding machine is advantageous in the workingof glass fiber filled polyester materials in that excessive working ofthe material does not occur and it would be desirable to utilize suchmachine in high-production application.

Such machines have been given consideration in the past but in order toachieve a reasonable degree of efficiency of operation, it is necessaryto incorporate a portion of the curing cycle with that of the shotpreparation cycle. The most convenient way of accomplishing this is toutilize a direct resistance heating technique for conductively elevatingthe temperature of the shot of material. Such a technique, of course,has drawbacks in that the elevating heat must be transmitted through themedium of the machine itself, and no close control over the endtemperature can be obtained. Further, since conduction occurs from theoutside of the shot toward the center a substantial temperature gradientoccurs within the material which must be accommodated in the finalcuring of the molded product and which introduces a variation in theworking of the shot while within the injection machine.

It would be desirable to provide apparatus which would allow a closecontrol of temperature levels in a shot-type injection molding machineand which can be utilized reliably as effecting a partial cure of theend product to reduce the cycling time of the machine.

Therefore it is a primary object of this invention to provide injectionmolding apparatus incorporating dielectric heating for elevating thetemperature of the shot material.

It is another object of this invention to provide an injection moldingmachine having a dielectric heater incorporated therein, which heater isintegrally related with the cycle of operation of the molding machine.

It is yet another object of this invention to provide an injectionmolding machine of the indexing type wherein dielectric heating isapplied in direct relation to position of an indexing member of themachine.

It is yet another object of this invention to provide injection moldingapparatus which advantageously employs dielectric heating in a shotpreparation stage of the machine wherein dielectric heating can occursimultaneously with preparation and indexing of the machine.

It is a yet further object of this invention to provide improvedelectrode arrangements for use in the dielectric heating portion of aninjection molding machine.

Dielectric heating is especially suited for utilization in the curing ofglass fiber fill polyester materials in that considerable control can beattained over the distribution of heat and temperature gradients withinthe material to avoid unwanted complete curing or inconsistent workingcharacteristics of the material. Such heating technique involves thesetting up of a field of energy in an electrode arrangement wherebymolecules of susceptible material exposed to such field are excited andobtain an elevated temperature. It is well known that extremely highfrequencies of operation and voltage levels must be attained in order tocreate a suitable field in this type of heating and that the electrodearrangement is significant in achieving a desired direction for thefield. In the past, however due to the extremely high-energy levels andthe difficulty of controlling sarne, little attention has been directedto the utilization of such heating energy in a manufacturing procedurewhich involves substantial inherent mechanical variation. However, thedesirable characteristics of such heating technique is so great inproducing an even distribution of heat within the material, commonlyreferred to as a heating effect occurring from within, that an improvedend product can be obtained by the combination of the dielectric heatingtechnique in the injection molding type of machine.

In the drawings:

FIG. 1 is a plan view with parts cut away of the injection moldingmachine of the invention with the shot cylinder oriented in theshot-receiving position;

FIG. 2 is an elevational view with parts cutaway of the apparatusdepicted in FIG. 1;

FIGS.. 3, 4 and 5 are isolated views of various electrode arrangementswhich may be utilized in the injection molding machine of FIGS. 1 and 2;

FIG. 6 is an elevational view of a further modification of theinvention;

FIG. 7 is an electrical schematic diagram of the control system; and

FIG. 8 is an electrical circuit diagram of the heating apparatus of theinvention.

Referring now to FIGS. 1 and 2, respectively being the plan andelevational views of the apparatus forming the injection molding machineof the invention, there is shown a shot frame 10 for support of thecomponents of the machine. Such shot frame is generally of open sidecubicle construction and supports therein a shot wheel member 11 forindexing between shot receiving and shot ejecting positions. Theapparatus of the invention is disclosed in greater detail in applicantscopending application Ser. No. 793,838 including a description of thesequence of operations and the functions of the components of themachine. For purposes of understanding the details of operation of thisinvention, however, only a general description of the interrelation ofcomponents is required from which the operation of this machine systemwill become clear.

The shot wheel 11 is mounted on a stub shaft I2 joumaled in a bearing 14in the shot frame for indexing movement between the material receivingand ejection positions. The wheel 11 is a cylindrical solid structureand is bored or otherwise apertured along a diameter thereof to form ashot cylinder 15 for receipt of the molding material. The wheel 11 isfabricated of some material which is not affected by the electricalfield of the dielectric heating apparatus to be described in more detailhereinafter and preferably is of a molded ceramic construction toprovide sufficient strength for withstanding the injecting pressures,although other insulative material such as Teflon would be suitable forthis application.

Also mounted on the stub shaft 12, for imparting movement thereto, is apinion 16 which cooperates with a rack 17 connected in turn to a doubleacting hydraulic ram 18. Indexing movement is then imparted to the wheel11 by the application of fluid under pressure to the hydraulic ram 18under direction of a pair of solenoid-actuated fluid valves, the endlimits of the wheel 11 being determined by the abutment of a stop rod 19mounted on the wheel with one of a pair of abutment surfaces 20 of theframe 10 in alignment with the stop rod 19. An electrical designation ofthe wheel II position is also provided through the cooperation of a pairof microswitch-type limit switches 21, 22 fixedly mounted with respectto the shot frame 10 for cooperation with a cam 24 carried by the rack17 and arranged for actuation when the wheel 11 is in either endposition.

The upper portion of the frame 10 is formed in the configuration of acylindrical chamber 25 having a lower conical surface 26 communicatingwith a passage 28 located in a downward protrusion 29 of the frame. Thepassage 28 is of cylindrical configuration matching that of the shotcylinder 15 and aligned therewith when the shot cylinder 15 is in thevertical or material receiving position as depicted in FIGS. 1 and 2.The chamber 25 is adapted for loading with the fiber-filled polyestermaterial to be used in the molding cycles and receives a squeeze piston30 mounted in a frame 31 at the upper portion of the shot frame andactuated by a hydraulic ram (not shown) in the controlled cycle to bedescribed.

A backup plunger 32 is provided in the lower portion of the shot frame10 being guided through a cylindrical opening in an upward protrusion 34of the frame for entrance into the shot cylinder 15. The backup plunger32 normally lies outside the confines of the wheel 11 but is moved to aposition within the wheel when the latter is in the shot-receivingposition as indicated in FIG. 2 by a solenoid-controlled ram 35. Intypical operation then, the backup plunger 32 is inserted in the shotwheel 11 and the squeeze piston 30 is actuated to force material throughpassage 28 into the shot cylinder 15, thereby forcing the backup plunger32 to a retracted position by means of a bleed arrangement in thehydraulic line of the backup ram 35. The position of the backup plunger32 within the wheel 11 and in a retracted position clear of the wheel isdetected by a pair of microswitches 36, 38 which may be mounted on anextension of the frame 10. It is clear that additional switches may beassociated with the backup plunger 32 to detect intermediate positionsthereof or the switches 36, 38 may be adjustably mounted, in order tovary the quantity of material introduced into the shot cylinder as willbe described with reference to the sequence of operation of the moldingmachine.

When the wheel 11 is indexed to the shot ejection position the shotcylinder 15 is then in alignment with a further pair of apertures in theshot frame 10, these being a first cylindrical passage 39 communicatingwith a die cavity 40 formed between movable and fixed dies 41 supportedon movable and fixed platens 42 in turn supported on the frame 10. Themovable platen 42 is actuated by a clamping mechanism (not shown) andthe die cavity 40, after receipt of the molding material, is subjectedto further heating for final curing of the material contained therein.

The second aperture 44 in alignment with the shot cylinder 15 guidesmovement of an ejection rod 45 which is transported through the shotcylinder 15 to displace the material from within the wheel 1] into thedie cavity 40, under actuation of a double-acting hydraulic ram 46, theend positions being detected by a suitable cam 48 and microswitch pair49, 50 associated with the rod 45.

In order to increase the efiiciency of the injection molding machine itis desirable to elevate the temperature of a quantity of the materialfrom the chamber 25 to bring about a partial curing which then iscompleted when the material has been injected into the die cavity 40. Inthe type of glass fiber filled polyester material with which thisinvention is primarily concemed, a polymerization temperature ofapproximately 320g may be considered as optimum for curing purposes toachieve desired characteristics in the finished end product. It has beendetermined that if the material is brought to a certain percentage ofthis optimum temperature, for example, approximately 280?, suitableworking characteristics of the material can be retained while achievingthe advantage of accomplishing the great portion of the curing cycleprior to injection into the die mold. In this embodiment of theinvention the advantages of dielectric heat may be utilized to theutmost wherein the heating strength may be concentrated at a desiredarea and a close control can be attained over the time of application ofheat and the effect of same upon the material.

The dielectric heating apparatus is depicted in a first form in FIGS. 1and 2 wherein a plate 51 of generally rectangular configuration ismounted within the shot frame 10 on a pair of insulating support members52, closely adjacent one side of the wheel 11 and generally in alignmentwith the shot cylinder 15 when the latter is in the injecting position,i.e., that position displaced from that depicted in FIGS. 1 and 2. An RFgenerator 54 is shown in FIG. 1 situated adjacent the shot frame 10 witha high-potential electrical connection being made to the plate 51 bymeans of a conductive rod 55. The rod 55 passes through the side of theshot frame 10 which is sufficiently apertured so as to provide adequateisolation for the high-frequency energy. Ground connection is made byway of a conductive conduit 56 connected between the RF generator 54 andthe shot frame 10, the conduit 56 serving to shield the connection toprevent radiation of the energy. Similarly, although not shown in thesedrawings, it is desirable to entirely enclose the shot frame 10 toprevent stray radiation. This may conveniently be done by the attachmentof thin conductive sheets over the open areas of the shot frame, whichis at ground potential thereby eliminating hazards to operatingpersonnel.

A second plate 58 of generally the same configuration as plate 51 ismounted on the stub shaft 12 and on the wheel 1! lying in alignment withthe shot cylinder 15 in all positions. The second plate 58 and the stubshaft 12 provide an electrical connection to the grounded shot frame 10and it is clear that when the wheel 11 is indexed to the injectposition, a substantially direct field of electric energy will beestablished between the plates 51, 58 passing through a portion of thewheel I] and encompassing the material contained in the shot cylinder15. The plates 51, 58 thus act as capacitor in the electrical outputcircuit of the RF generator 54, providing a maximum capacitancecondition when in parallel alignment and a minimum capacitance when at90 to one another. It will be apparent then that if the output circuitis tuned to provide a maximum transfer of power between the plates 5 1,58 when in the inject position that movement of the shot wheel 11 to thereceiving position will cause a detuning of the circuit and an automaticdiminution of the power from the RF generator 54, thereby providing anautomatic control of the output power level dependent upon position ofthe shot wheel 1 l.

Tuning of the output circuit of the RF generator 54 may be accomplishedin various ways including an adjustment of the plate 51 with respect tothe shot frame 10 in that there is a capacitance effect between suchhigh potential and grounded potential areas. Inductance in the outputcircuit is contributed by the length of connecting rod 55, thehigh-potential plate 51 and a tuning stub 60 comprising a length ofconductive tubing connected to the rod 55 at one end and at the otherend to the shot frame 10. At the high frequencies of operation of the RFgenerator which typically is in the range of megacycles, such inductiveeffect is readily realized.

Referring now to FIG. 3, another embodiment of the electrode arrangementand coupling for the RF generator is shown wherein parts correspondingto those previously described are identified with corresponding numeralswith the subscript a appended thereto.

In this embodiment of the invention the shot wheel Ila is shown in theshot-injecting position, again mounted on a stub shaft 12a along withthe grounded plate 58a of the output load circuit for indexing movementby the rack mechanism. The high-potential plate 51a is mounted in fixedrelation to the shot frame 10a on a pair of insulators 52a and aspreviously noted such plates 51a, 58a may comprise rectangular plateshaving a width approximating the diameter of the shot cylinder 15a oralternatively may comprise completely circular plates approximating theperiphery of the shot wheel lla so that RF energy is continuouslycoupled therebetween and not dependent on wheel position. The field ofenergy between the plates is depicted by a series of dashed arrows 61indicating the generally straight field of energy developed in thiselectrode arrangement.

In this embodiment an auxiliary plate 62 is mounted on the shot frame ona pair of insulating standards 64 which comprise threaded membersreceived in nuts 65 fixed to the shot frame and rotatably mounted in theplate 62 so that adjustment of the position of the plate 62 relative toplate 51a may be made. Electrical connection is made to the auxiliaryplate 62 by a flexible cable 66 leading to the RF generator which allowsa freedom of movement. The auxiliary plate 62 and the electrode plate51a thus form a capacitor for variably coupling the energy from the RFgenerator into the material in the shot cylinder a. Such arrangementalso allows a convenient manner for tuning the output load circuit ofthe RF generator as well as for affecting the quantity of energy coupledto the ground plate 58a.

Although a relatively evenly distributed electric field is developed inthis electrode arrangement further efficiencies can be realized byvarious other electrode configurations commensurate with an evenness ofheating throughout the shot cylinder.

Thus in FIG. 4 yet another embodiment of electrode arrangement isdepicted wherein both the energized and grounded plates 51b, 58b areformed of segments of conductive and insulating material arrangedlinearly along the shot cylinder 15b, the conductive segments 68, 69being electrically tied together through respective conductive backupplates 70, 71. The conductive segments 68 of the energized plate 51b arearranged to lie opposite the nonconductive segments 72 of the groundplate 58b so that electric fields indicated by the arrows 74 areestablished in a more longitudinal sense than the direct transversearrangement depicted in FIG. 3.

Such arrangement allows the same amount of material contained within theshot cylinder 15b to be exposed to a less intensive electric field thanrealized in the FIG. 3 embodiment but having greater length of traversewhereby a greater efficiency of utilization of RF energy is realized. Itwill be noted that an end effect occurs here also, as it does in theFIG. 3 embodiment, wherein part of the electric field is directed togrounded objects closely abutting the shot wheel 11b as indicated by thedashed end arrows 75 in FIG. 4. The extended length of energy fieldhowever assures an even distribution of heat throughout the shotcylinder 15b and will tend to minimize the efiect of directing anelectric field through a cylindrical member from a pair of parallelplane plates 51a, 58a as in FIG. 3, wherein the center portion of theshot cylinder 15 may be exposed to an additional heating effect.

In FIG. 5 yet another embodiment of electrodes for the dielectricheating arrangement is depicted in a perspective view of the shot wheel11c. In this embodiment of the invention the shot wheel 110 may bemounted in the manner described in FIGS. 1 and 2, but in thisarrangement a stray electric field is developed within the shot cylinder15c as opposed to the direct electric fields realized in the FIGS. l-4embodiments.

The electrodes are arranged as a pair of energized electrodes 76 and analternate pair of grounded electrodes 78 equally arranged about theperiphery of the shot cylinder 15c and embedded in the shot wheel 11c.The electrodes 76, 78 are bars of conductive material extending the fulldiameter of the shot wheel 11c, having arcuate inner surfaces adjacentthe shot cylinder, thereby forming a portion of the shot cylinder 15c.The energized electrodes 76 are electrically connected to one another bymeans of a conductive strap 79 and a pair of conductive rods 80 threadedinto the electrodes. The strap 79 in turn is connected to the output ofthe RF generator by a flexible cable 81 to accommodate the indexingmovement of the shot wheel. The grounded electrodes 78 being exposed onthe flat surfaces of the shot wheel 11c are coupled in any convenientmanner and electrical connection to the shot frame may be made throughstub shaft as in the FIGS. l-4 embodiments.

The effect of this electrode arrangement then is to establish electricfields generally of the character indicated by the curved arrows 82 inFIG. 5, such electric fields minimally transversing the central portionof the shot cylinder but having an overlapping effect at this point tocreate an even distribution of electric energy in the shot cylinder 15c.It is apparent that any number of electrodes 76, 78 may be arranged in a5 jacent the material contained within the shot cylinder 15c therebyeliminating the airspaces inherent in the FIGS. 1-4 embodiments andallowing the coupling of a great quantity of the RF energy into thematerial in the shot cylinder.

Referring now to FIG. 8 there is shown an electrical schematic diagramof the RF generator and electrode arrangement for producing thedielectric heating energy in all embodiments of the invention. Suchcircuit is essentially an oscillator circuit capable of operating in themegacycle frequency range and may take many different forms this beingbut one example of a utilizable circuit.

Typically power is received from an AC source of supply 84 and iscoupled to the primary winding 85 of a power transformer through a pairof normally open contacts 86 which may be remotely controlled as will bedescribed in more detail hereinafter. The voltage of the secondarywinding 88 of the transformer is rectified by a diode 89 and filtered incapacitor 90 to provide a DC output at terminal 91. The output voltageis applied to the oscillator circuit 92 through an RF choke 94, whichprevents the RF oscillations from reaching the diode 89, and is appliedto the plate electrode of a high-frequency oscillator tube 95. Typicallythe tube 95 has the cathode connected directly to ground and the gridelectrode connected to a tuned circuit comprising series-connectedinductors 96, 97 and capacitor 98 in parallel connection across thelatter inductor 97.

The plate of the tube 95 is connected via a coupling capacitor 99 to thetuned load circuit 100 comprising effectively a parallel-connectedinductor 101 and capacitor 102 (as previously mentioned the inductor 101may comprise a tuning stub 60 located within the shot frame 10 or may beinherently realized in the form of apparatus), these being engineeringconsiderations well understood in the high-frequency field of art. Thecapacitor 102 in the tuned load circuit 100 represents the capacitiveeffect produced by any of the electrode arrangements described hereinand it will be apparent that the working energy of the circuit isrealized between the plates of the capacitor.

The oscillator 92 thus comprises a tuned load, tuned grid circuit, thetube 95 having sufficient interelectrode capacitance to sustainoscillation and it is one advantage of this invention that such circuitcan be rapidly switched between oscillating and nonoscillatingconditions to accurately control the energy applied to the electrodearrangements. Conventionally the filament (now shown) of the oscillatortube 95 will be energized from a suitable power source prior toapplication of anode power to avoid damage to the tube 95 as is wellknown in this art and will be considered as continuously energized forpurposes of this description.

Referring now to FIG. 7, there is shown a circuit for completelycontrolling the cycle of operation of the injection molding machinerequiring only a stepping switch 105, a plurality of solenoids 106 forcontrolling the various functions together with interlocking andposition-determining switches. Numerals corresponding to the componentsdepicted in FIGS. 1 and 2 with appropriate letters appended will beutilized in order to aid in an understanding of the control system andthe description of operation of the cycle will be related to the samefigures.

The stepping switch comprises a solenoid coil 108 which effectssimultaneous stripping of a pair of sliders 109, 110 in two decks ofcontacts. The solenoid coil 108 is connected at one end to ground andreceives power from an input terminal 111 by way of an array of contactsand switches. All solenoids 106 are connected in common to ground andreceive energizing power from the slider 110, connected in turn to aninput terminal 112 receiving a source of power, energization again beingcontrolled by a plurality of switches and contacts.

The cycle of operation is initiated then with the stepping switch in thelast or lowermost position with power applied to the input terminals111, 112. The shot wheel 11 is in the material-receiving position andmicroswitch 22 is actuated by cam 24. Contact 22x of microswitch 22 isin the now-closed position and the circuit is in preparedness forinitiation of the cycle. The cycle start switch 114 may be manuallyactuated to apply power to the solenoid coil 108 thereby moving thesliders 109, 110 to the first position as depicted in FIG. 7. Cyclestart switch 114 may be of the spring-loaded type if only single cyclesof operation are required wherein closure of the switch 114 is necessaryfor each cycle of operation or alternatively may remain in a closedposition for continuing cycles of operation. In the first position poweris applied to solenoid 35F through normally closed contact 36y ofmicroswitch 36, the solenoid 35F applying fluid to the backup rod ram 35to cause movement of the rod 32 into the shot cylinder 15. Upon reachingthe upper limit of movement microswitch 36 is actuated to open contact36y to remove power from solenoid 35F and thus ram 35, and close contact36x applying power to the solenoid coil 108 to cause stepping of thesliders 109, 110 to the second position.

In the second position the ram controlling the squeeze piston 30 isenergized through contact 38y thereby forcing material through passage28 and into the shot cylinder 15. The backup rod 32 is forced backwardlyvia the bleed arrangement for the ram 35 until microswitch 38 isactuated opening contact 38y, thereby removing power from the ram, andclosing contact 38x to cause further stepping of the stepping switch105.

A similar operation will be obtained for indexing of the shot wheel 11to the forward or shot-ejecting position whereupon in the fourthposition of the stepping switch 105 the heater solenoid 1 will beenergized. Several alternatives are available in this point of the cycleof operation including the automatic energization of the high-frequencyheat as previously described with respect to indexing movement of theshot wheel and wherein no heater solenoid would be required in thesequence of operations. Preferably, however, highfrequency heat ispositively controlled as an independent step in the cycle of operationand two alternatives are feasible once the heater solenoid 1 15 has beenenergized.

A heat-sensing element 116 is mounted on the shot wheel closely adjacentthe shot cylinder 15 so as to sense the elevation in temperature of thematerial in the shot cylinder and operative to control actuation of thecontacts 115a, b. Such element 116 is preferably a thermistor embeddedin the shot wheel 11 having a fairly fast response time and beinginsensitive to the high-frequency energy. The thermistor is connected toauxiliary electrical circuitry for actuating contacts 115a, b byflexible conducting leads (not shown). Alternatively, the heatersolenoid 115 may be a relay of the time-delay type wherein apredetermined time interval for energization of the heater and actuationof contacts 115a, b may be automatically effected. In either eventcontacts 86 in the primary circuit of the RF generator of FIG. 8 areresponsive to solenoid 115 and will be closed for an interval of time toapply power to the output circuit 100 including the electrodes 51, 58.

At the completion of the heating interval the remaining steps of thecycle of operation will take place whereby the eject ram 45 will bemoved to a forward and then reverse position, relying on the operationof contacts 49x, 49y and 50x, 50y, and the sliders 109, 110 willsequence to the last position of the stepping switch 105. At this pointthe shot wheel 11 will be indexed in a reverse direction to theshot-receiving position, solenoid 18R and thus ram 18 being deenergizedby the opening of contact 22y and as previously mentioned the cycle willstop here or will be automatically continued depending on the closure ofthe cycle start switch.

Referring now to FIG. 6, there is shown yet another embodiment of theinvention wherein the dielectric heating is applied to the materialprior to its introduction into the shot wheel 119 so that a simultaneousheatin of the material and injection of the previous shot into the recavity 40 may be performed resulting in a substantial reduction incycling time. In this embodiment of the invention the shot frame 118 ismodified so that cylindrical chamber 120 for retaining the polyestermaterial and for accepting the squeeze piston 121 is elevated asubstantial distance above the shot wheel 119. A tube 122 of insulatingmaterial, as the ceramic previously mentioned, is affixed to the shotframe 118 and forms a passage between the chamber 120 and the shotcylinder 125, perfonning the function of the protrusion 29 described inrelation to the FIGS. 1 and 2 embodiment.

A pair of conductive plates 126, 127 of approximately the width of thetube 122 are supported on either side of the tube 122 and form theelectrodes for the dielectric heating system, plate 126 beinginsulatively supported and connected to the energized output of the RFgenerator and plate 127 being connected to ground.

Thus it will be clear in the cycle of operation that when material isforced into the shot cylinder against the backup rod, as previouslydescribed, a definite amount of the material will be left in the tube122 and may be acted upon by the dielectric heater during the indexingof the shot wheel 119. The length of the tube 122 should be identical tothe diameter of the shot wheel 119 when the passage 124 therein and theshot cylinder 125 are of the same diameter, but it is clear thatvariations may be effected in the apparatus so that it is only necessaryto expose the same amount of material to the dielectric heating energyas will be contained within the shot cylinder 125 on the succeedingcycle. It is clear also that the cycle control system depicted in FIG. 7will be modified in this embodiment of the invention but such adaptationwill be well within the skill of the person familiar with this art.

I, therefore, particularly point out and distinctly claim as myinvention:

1. An injection molding machine of the shot type comprising an indexablemember having a shot cylinder therein for receipt and discharge of aquantity of plastic material, said member being supported for rotationabout an axis passing through said shot cylinder between a firstshot-receiving position and a second shot-injecting position, a firstelecnode mounted adjacent said indexable member at one side of said shotcylinder, a second electrode mounted on said indexable member at theopposite side of said shot cylinder, said first and second electrodesdefining a field for electric energy through said shot cylinder whenbrought into alignment with one another by rotation of said indexablemember, and a radiofrequency generator operatively interconnected withsaid first and second electrodes for creating an electric field ofheating energy therebetween to elevate the temperature of the plasticmaterial for a partial cure prior to injection.

2. A machine as set forth in claim 1 wherein said first and secondelectrodes are plates of conductive material, having dimensionsconforming substantially to those of said shot cylinder, and adapted tobe parallel one another for defining an electric field through said shotcylinder only when said indexable member is in the shot-injectingposition.

3. A machine as set forth in claim 1 wherein each of said first andsecond electrodes comprises a plurality of equally spaced conductivemembers, the members of said first electrode being alternately disposedin relation to the members of said second electrode and all said membersbeing substantially equally disposed adjacent the shot cylinder tocreate multiple paths of heating energy through the shot cylinder, meansfor grounding the members of said second electrode, and means forinterconnecting the members of said first electrode.

4. A machine as set forth in claim 3 wherein the members of said firstelectrode are linearly aligned at one side of the shot cylinder and themembers of said second electrode are linearly aligned at the oppositeside of the shot cylinder thereby to effect transverse electric fieldsthrough the shot cylinder.

1. An injection molding machine of the shot type comprising an indexablemember having a shot cylinder therein for receipt and discharge of aquantity of plastic material, said member being supported for rotationabout an axis passing through said shot cylinder between a firstshot-receiving position and a second shot-injecting position, a firstelectrode mounted adjacent said indexable member at one side of saidshot cylinder, a second electrode mounted on said indexable member atthe opposite side of said shot cylinder, said first and secondelectrodes defining a field for electric energy through said shotcylinder when brought into alignment with one another by rotation ofsaid indexable member, and a radiofrequency generator operativelyinterconnected with said first and second electrodes for creating anelectric field of heating energy therebetween to elevate the temperatureof the plastic material for a partial cure prior to injection.
 2. Amachine as set forth in claim 1 wherein said first and second electrodesare plates of conductive material, having dimensions conformingsubstantially to those of said shot cylinder, and adapted to be parallelone another for defining an electric field through said shot cylinderonly when said indexable member is in the shot-injecting position.
 3. Amachine as set forth in claim 1 wherein each of said first and secondelectrodes comprises a plurality of equally spaced conductive members,the members of said first electrode being alternately disposed inrelation to the members of said second electrode and all said membersbeing substantially equally disposed adjacent the shot cylinder tocreate multiple paths of heating energy through the shot cylinder, meansfor grounding the members of said second electrode, and means forinterconnecting the members of said first electrode.
 4. A machine as setforth in claim 3 wherein the members of said first electrode arelinearly aligned at one side of the shot cylinder and the members ofsaid second electrode are linearly aligned at the opposite side of theshot cylinder thereby to effect transverse electric fields through theshot cylinder.